In an information recording medium such as Blu-ray disc, a recording layer is formed on a substrate, and this construction enables writing or reading of data by converging light on the recording layer.
Further, a track of spiral shape is formed on the medium, and reading and writing of data is carried out by converging laser beam along the track while the information recording medium is placed on and rotated by e.g. a spindle motor.
The track is realized by a groove or pits physically engraved on a substrate. For example, in a recordable Blu-ray disc, a track was realized by a groove. Further, for example, in a read only type Blu-ray disc, a track was realized by pits.
Here, a groove means a concave-convex pattern physically and continuously formed along a circumferential direction of an information recording medium. Further, pits mean concave-convex patterns discontinuously formed in a circumferential direction of an information recording medium, and a plurality of pits arranged in the circumferential direction of the information recording medium constitutes a track.
Here, some type of information recording medium has both a groove and pits engraved in respective regions of a single information recording medium. Namely, there is a case where a region in which only a groove is engraved is used as a recordable region and a region in which only pits are engraved is used as a read only region.
Meanwhile, in a case of recordable Blu-ray disc, e.g. the most suitable recording power of the information recording medium or a media manufacturer information are recorded on the information recording medium as control data in advance (a region in which control data is recorded is referred to as “control data region”, and it may also be referred to as “PIC region” as described later.). Recording of control data is achieved by wobbling a groove by a modulation method different from that in a user data region.
Here, wobbling is achieved by displacing a groove in a very small amount in the radial direction of an information recording medium with a predetermined amplitude and pattern. Further, the modulation method is a conversion method for converting data that is desired to be recorded in advance, into a predetermined wobble pattern. Here, in the following description, for simplification, “a modulation method used for wobbling a groove” is simply referred to as “wobble modulation method”.
Further, in a recordable Blu-ray disc, in order to ensure reading of control data by a drive, a track pitch in the control data region is formed larger than a track pitch in the user data region.
Here, a track pitch means a distance between centers of adjacent to tracks when the information recording medium is observed in the radial direction. As described above, in a recordable Blu-ray disc, the groove is wobbled in the radial direction of information recording medium by a small amount, but at a time of discussing the value of track pitch, wobble of the groove is not considered. Accordingly, the track pitch is equal to an average distance between grooves when the information recording medium is observed in the radial direction.
A laser beam irradiated on the information recording medium is partially reflected, and the reflected beam is received in e.g. a two-segment photodetector.
FIGS. 15(a) to 15(c) are graphs showing correlations between normalized push-pull signal amplitude (amplitude of signal representing the difference between outputs of segments of the two-segment photodetector whose detection plane is split in a direction along a track, divided by the sum of these outputs) and a track pitch, a groove depth and a groove width, respectively.
It is understandable from FIGS. 15(a) to 15(c) that in e.g. a case of recordable Blu-ray disc, the track pitch in the control data region is wider than the track pitch in the user data region, and accordingly, when the groove shapes in the control data region and the user data region are substantially the same, the normalized push-pull signal amplitude obtained from the control data region is larger than the normalized push-pull signal amplitude obtained from the user data region. Further, it is known that the amplitude of normalized push-pull signal changes depending on the groove width and groove depth.
A pickup having an objective lens for converging laser beam follows a track based on the normalized push-pull signal (hereinafter operation of following a track is referred to as “tracking”, and control for performing tracking is referred to as “tracking servo”.). A pickup is usually designed on the assumption that a normalized push-pull signal amplitude is within a predetermined range. In other words, it is necessary that the normalized push-pull signal amplitude is within a predetermined range in order to realize stable tracking servo.
For example, when the normalized push-pull signal amplitude is smaller than the predetermined range, sufficient signal amplitude for realizing stable tracking servo can not be obtained, and there occurs a problem of tracking servo error.
Further, for example, when the normalized push-pull signal amplitude is larger than the predetermined range, too high signal amplitude is input into a tracking servo circuit, and as a result, there occurs a problem that the servo system oscillates.
Further, when the normalized push-pull signal amplitude is relatively too large, a focus error signal to be used for focus servo overlaps with a push-pull signal (a differential signal of outputs of segments of two-segment photodetector whose detection plane is split in a direction along a track), which may cause a problem such as focusing error.
Here, some type of pickup uses a signal other than a normalized push-pull signal to carry out tracking, but it is common even to such a pickup that the signal amplitude used for carrying out tracking is required to be within a predetermined range to realize stable tracking servo.
Further, even when the normalized push-pull signal amplitude is within a predetermined range, if quick change of normalized push-pull signal amplitude occurs, a gain control circuit for servo system can not follow the change, and tracking servo error may be caused. Accordingly, it is advantageous to reduce change of the normalized push-pull signal amplitude as much as possible in order to reduce the possibility of occurrence of trouble of tracking servo.
As described above, since quick change of normalized push-pull signal adversely affects tracking servo or focus servo, for example, it is known to be advantageous to make the groove shapes in the control data region and the user data region different from each other, and thereby reduce the difference between normalized push-pull signal amplitudes in the control data region and the user data region, to prevent such an affect.
Here, according to a common production method of information recording medium, it is possible to make groove shapes of the regions different by making exposure powers of master exposure apparatus for the regions different from one another.
Further, in order to realize stable tracking servo in the regions having different tracking pitches, it is proposed to constitute a single spiral-shaped track by regions having different track pitches, and to provide a transition section in which the track pitch gradually changes (Patent Document 1).
The above technique of providing a transition section in which track pitch gradually changes, is used, for example, in a recordable Blu-ray disc. According to this technique, a control data region and a user data region form a continuous spiral-shaped track, and these regions are connected by interposing a track pitch transition section Stp in which track pitch gradually changes.
Arrangement of the control data region, the user data region and the track pitch transition section Stp in a recordable Blu-ray disc is described with reference to FIG. 16. Here, FIG. 16 is a view for explaining arrangement of the control data region, the user data region and the track pitch transition section Stp in a common recordable Blu-ray disc.
As shown in FIG. 16, in a recordable Blu-ray disc, a control data region is arranged more inside from a user data region, and the track pitch in the control data region is set to be about 0.35 μm, and the track pitch in the user data region is set to be about 0.32 μm.
Further, between the user data region and the control data region, a protection zone is provided, and the track pitch transition section Stp is formed so as to be completely contained in the protection zone.
The track pitch in the protection zone (inner peripheral side protection zone in FIG. 16) present more inside from the track pitch transition section Stp, is set to be the same as the track pitch in the control data region.
On the other hand, the track pitch in a protection zone (outer peripheral side protection zone in FIG. 16) present more outside from the track pitch transition section Stp, is set to be the same as the track pitch of the user data region.
Further, the wobble modulation methods in the control data region and the user region are different, and the wobble modulation method in the protection zone is the same as the wobble modulation method in the user data region. Namely, the wobble modulation method changes at a border between the protection zone and the control data region (Patent Document 2).
Namely, the protection zone is a zone that is provided between two regions between which the wobble modulation method and track pitch are different, and provided for connecting the two regions formed in a single spiral-shaped track.
Patent Document 1: JP-A-2003-346384
Patent Document 2: JP-A-2006-12355